Abstract : In the framework of a geological disposal of radioactive waste, cement-based materials will be highly used; the deep geological disposal facilitywill imply large amounts of concrete in the clayrock formation. In this context, low pH cementitious materials are considered, especiallyfor sealingrequirements, in order to minimize chemical interactions at the interfaces between cementitious materials and the clay surrounding rock formations and/or engineered clay materials. The target is to reach a concrete pore solutionpH more compatible with the clay materials or rock formation (pH ~7-7.5) for reducing the detrimental impact of the alkali plume from the cement-based materials. Recent studies 1–3 have proposed low-pH formulations based on ternary and quaternary mixes of Portland cement with supplementary cementitious materials (SCMs) such as blast furnace slag (BFS) andfly ash (FA).For these formulations, the pHis controlledby low C/S C-S-H. However, if forOrdinary Portland Cement (OPC) concrete formulations, the hydration products are well characterized with suitable constrained kinetic/thermodynamic models 4 [4], , the mineralogical control of elements in solution have to be discussed for low pH formulations because of their higheraluminum and magnesium contents 5. In particular, for the hydration of MgO, which is mainly introduced by the BFS, Zhang et al. 6 have mentioned the precipitation of brucite (MgOH 2) which reacted with the silica fume to produce a magnesium silicate hydrate (M-S-H). The calculated pH in equilibrium with this mineralogical assemblage is around 10.5 and satisfies the requirement of low pH condition. The occurrence of M-S-H has been mentioned in many environments 6–9 , and was described as low crystalline phases according to the low intensity and broad peaks signals. Despite this abundance of M-S-H evidences and characterizations, the structure of M-S-H is not well known. Some authors have explained the precipitation of such Mg-silicate hydrates according to Ca/Mg isomorphic substitution in the calcium silicate hydrates (C-S-H). The present study aims to determine this crystal structure and to define their nature (cement phase or phyllosilicate). Two low temperature synthesis of M-S-H have been made, with Mg/Si ratios of 0.6 and 1.2, close to the talc composition (Mg 3 Si 4 O 10 OH 2) and consistent to the Ca/Si ratio of the C-S-H phases. Crystal chemistry of M-S-H was determined by combining TGA, electron probe micro-analysis (EPMA), TEM, NMR and powder X-ray diffraction. These experimental investigations of poorly crystalline Mg-silicates showed a 2:1 magnesium phyllosilicate-like structure with short range stacking order, identified as a talc-like structure. All the talc structure crystallographic evidences described in previous studies are in agreement with the characterization of the talc synthesis performed for short synthesis time